Remove support from end-of-life Python 3.6

(In anticipation of possibly adding type support)

docs/source/install.rst

@@ -5,7 +5,7 @@ To install panflute from PyPI, open the command line and type::
 
     pip install panflute
 
-- Works with Python 3.6+, and PyPy
+- Works with Python 3.7+, and PyPy
 - On Windows, you might need to open the command line (``cmd``)  as administrator (`ctrl+shift+enter`).
 
 To install the latest Github version of panflute, type::

setup.py

@@ -70,7 +70,6 @@
         # Specify the Python versions you support here. In particular, ensure
         # that you indicate whether you support Python 2, Python 3 or both.
         # https://pypi.python.org/pypi?%3Aaction=list_classifiers
-        'Programming Language :: Python :: 3.6',
         'Programming Language :: Python :: 3.7',
         'Programming Language :: Python :: 3.8',
         'Programming Language :: Python :: 3.9',
@@ -87,7 +86,7 @@
     # simple. Or you can use find_packages().
     packages=find_packages(exclude=['contrib', 'docs', 'tests', 'examples']),
 
-    python_requires='>=3.6',
+    python_requires='>=3.7',
     # List run-time dependencies here.  These will be installed by pip when
     # your project is installed. For an analysis of "install_requires" vs pip's
     # requirements files see: